1. Field of the Invention
The present invention relates to a multi-functional sensor and, more particularly, pertains to a design of a reliable capacitive oil deterioration and level sensor whose measurements are insensitive to changes in the operating environment of the sensor.
2. Description of the Related Art
One of the most important factors that contributes to the efficiency and durability of internal combustion engines is lubrication. As engine oil "breaks down" it is less effective in protecting an engine from damage caused by friction between engine parts. More specifically, the normal life span of motor oils is limited by thermal breakdown, additive depletion and carbon particulates that result from the combustion process.
The deterioration of engine oil is marked by a decrease in the viscosity of the oil. The dielectric constant of engine oil provides an indication of the oil's deterioration or lack thereof.
The dielectric constant of motor oil is typically between 1.6 and 3.2 depending upon its brand and age. For example, the dielectric constant of a particular brand of motor oil may increase from 2.19 to 3.2 after 400 hours of use in a particular internal combustion engine under certain operating conditions. Thus, it is desirable to have a means for indicating reliably when engine oil has deteriorated to the point where it should be changed.
Premature lubrication failure can also result from the presence of contaminants in the engine oil such as coolant (glycol ethylene), filel or water. The presence of these contaminants in motor oil is often indicative of a mechanical failure such as a damaged head gasket or a broken piston ring. Water and engine coolant have dielectric constants of approximately 87.5 and 37.0, respectively. The introduction of such contaminants into the engine oil significantly increases the dielectric constant of some of the fluid which circulates through the engine for the purpose of lubrication. Thus, it is also desirable to have a means for detecting the presence of these substances in engine oil.
The dielectric constant of oil is also influenced by the temperature of the oil and by the specific formulation of a given brand of oil. Thus, it is also desirable to provide a multi-functional sensor with a means for compensating for changes in the dielectric constant of oil resulting from these factors.
U.S. Pat. No. 5,929,754 of Dr. K. M. Park et al. discloses a high-sensitivity capacitive oil deterioration and level sensor that measures the dielectric constant of the liquid suspension. However, under certain circumstances, the output of this sensor has not been as stable as would be desired.
Accordingly, a principal object of the present invention is to increase the stability and reliability of sensors of the type disclosed in the above-identified patent.